1. Field of the Invention
The invention relates to a receiver/decoder for the reception of digital data transmitted by an RF carrier signal using a modulation technique known as frequency shift keying, referred to in this disclosure as FSK. More particularly it relates to such a receiver/decoder which can function as the transmitting source for an FSK modulated signal in a format identical to that received.
2. Background of the Invention
In a system using a known FSK receiver/decoder, referred to herein as the known FSK receiver, two appropriately separated frequencies f.sub.1 and f.sub.2 are used to transmit information by FSK.
In such system using the known FSK receiver, digital information can be transmitted by designating frequency f.sub.1 as the presence of data bit 1, and f.sub.2 as the presence of data bit 0. A continuous digital data stream can be transmitted by changing between the frequencies f.sub.1 and f.sub.2 during regular, predetermined sampling intervals known as time slots.
The known FSK receiver senses the presence of frequencies f.sub.1 and f.sub.2 in successive time slots and appropriately assigns data values of one or zero according to the received frequency. The digital data stream thus received can be assembled into data blocks for storage, processing and/or later retrieval.
A disadvantage of this method of data modulation and transmission is that all possible combinations, i.e., each of multiple possible states, of frequencies f.sub.1 and f.sub.2 occur in the digital sequences 01, 00, 10, and 11. The data speed or rate, defined as the ratio of the number of transmitted frequencies to the number of data bits transmitted, is therefore unity, since only two frequencies are used to signal the data bits 1 and 0. In known FSK signal transmitters and receivers, this disadvantage may be overcome by transmitting frequencies f.sub.1 and f.sub.2 in the microwave range. By using extremely short duration time slots, and high transmitter frequencies, high transmitted data rates can be achieved.
Such technique is not advantageously employed where the system, for other reasons, is constrained to much lower transmitter frequencies, such as in the VLF frequency region, for the purpose of minimizing transmitter directional effects and to achieve extreme economy in the outlay and power use of decoding circuitry.
Accordingly, among the several objects of the present invention are to provide an FSK receiver/decoder which is capable of the highest possible data rates in applications where transmitter frequencies are constrained to much lower values than the microwave range, e.g., in the VLF region; which is useful in a system in which transmitter frequencies are relatively low for the purpose of minimizing transmitter directional effects; and which provides highest possible data rates while achieving extreme economy in outlay and power use.
A further object of the invention is to provide such an FSK receiver/decoder which permits it to be easily reconfigured as a transponder for operation in a mode in which data can be generated for retransmission in a format identical to that received.
It is also an object of the invention to provide such an FSK receiver/decoder which is operable at relatively low frequencies while employing more than two multiple transmitted frequencies, viz., preferably four discrete frequencies, each coded to two bits over four time slots, whereby to double the data speed over that of known two-frequency systems with two time slots; and which advantageously minimizes and simplifies the encoding circuitry when the receiver/decoder operates in the transponder mode.
It is also an object of the invention to provide an FSK receiver/decoder capable of for overcoming a temporary loss or fade of receive input signal strength resulting from environmental conditions.
A further object of the invention is to provide such an FSK receiver/decoder of high efficacy in a system.